Add attribute-based indices to the tf_objects table to speed up the search

[dtitov]

Is your feature request related to a problem? Please describe.
The most time-consuming part of the search is actually the execution of a search-query. For example,
11 seconds for experiments.content->'technique'->'term_id' ? 'http://purl.obolibrary.org/obo/OBI_0000748' query and no limit on entries.

Describe the solution you'd like
We should consider adding indices on attributes, e.g. CREATE INDEX idx_tech_type ON tf_objects (content->'technique'->'term_id'); or smth like that. The list of the attributes to index can be extracted from the tf_metamodel table.

Describe alternatives you've considered
We may consider putting different object types to different tables, but it implies a huge overhead in terms of the development - not desirable.

Additional context

[sveinugu]

Priority, but after #101

[amgadhanafy]

dev server trackfind-dev.gtrack.no

[dtitov]

@amgadhanafy, any progress on this?